Why is it that some people feel perfectly justified to occupy a seat while an elderly man or largely pregnant woman stands? Worse, this troll might be actually sitting in a seat beneath a sign that says "reserved for the elderly" and feel no compunction about it.

This is just bad karma. Imagine if it was you who was injured, pregnant or elderly, and no one offered you a seat? Granted, not everyone with these conditions wants to sit but you won't know unless you ask them. You'll feel so much better about yourself (and possibly earn a smile) if you do the right thing.